1. Field of the Invention
This invention relates to alkaline fuel cells and alkaline batteries. In one aspect, this invention relates to alkaline fuel cells and alkaline batteries which are tolerant to CO2, a known “poison” to the alkaline electrolyte of alkaline fuel cells and alkaline batteries. In one aspect, this invention relates to alkaline fuel cells and alkaline batteries employing polymeric anion exchange alkaline electrolyte membranes. In one aspect, this invention relates to CO2 tolerant alkaline electrolyte membrane fuel cells and batteries. In one aspect, this invention relates to CO2 tolerant polymeric anion exchange alkaline electrolyte membranes.
2. Description of Related Art
Alkaline fuel cells produce power through a redox reaction between hydrogen and oxygen. At the anode electrode, hydrogen is oxidized according to the reactionH2+2OH−→2H2O+2e−producing water and releasing two electrons. The electrons flow through an external circuit to the cathode electrode at which oxygen is reduced according to the reactionO2+2H2O+4e−→4OH−producing hydroxide ions. The net reaction consumes one oxygen molecule and two hydrogen molecules in the production of two water molecules. Electricity and heat are formed as by-products of this reaction.
In a conventional alkaline fuel cell, the cathode and anode electrodes are gas diffusion electrodes comprising a catalyst layer, a catalyst support layer, and a gas diffusion layer, and the electrodes are separated by a porous matrix, e.g. a nylon sponge, saturated with an aqueous alkaline solution, such as potassium hydroxide (KOH). Aqueous alkaline solutions do not normally reject carbon dioxide (CO2) as a result of which the fuel cell is easily “poisoned” through the progressive carbonation of the solution in accordance with the reactionCO2+2OH−→CO3−2+H2OThis reaction leads to a decrease in the overall performance of the alkaline fuel cell overtime. In addition, carbonate precipitation in the cathode impairs the performance because of the three-phase boundary,
In the operation of a conventional alkaline fuel cell, air is provided to the gas diffusion layer of the cathode through which it is transmitted to the catalyst support layer and then the catalyst layer, which is saturated, for example, with KOH solution. If the air contains CO2 or if fuel oxidation produces CO2, the CO2 will react with the KOH to form K2CO3 in the catalyst layer, producing the following effects: a) reduction in the OH− concentration and fuel oxidation/oxygen reduction kinetics; b) increase in the electrolyte viscosity resulting in lower diffusion coefficients and lower limiting currents; c) the eventual precipitation of carbonate salts in the pores of the porous electrode; d) reduction in oxygen solubility; and e) reduction in electrolyte conductivity. A similar mechanism applies to alkaline batteries, especially air-metal batteries.
Alkaline membrane fuel cells, in which the electrolyte is an anion exchange membrane, offer significant advantages over conventional aqueous alkaline-solution based alkaline fuel cells in that membrane based systems avoid issues of electrolyte migration, mitigate corrosion concerns, can be operated with differential pressures, and offer design simplification. Most significantly, alkaline membrane fuel cells are able to operate without the noble metal catalysts employed in conventional alkaline fuel cells. However, low OH− conductivity, water swelling, and chemical stability are problematic with conventional alkaline exchange membranes. Low OH− conductivity of conventional anion exchange alkaline membrane fuel cells has been addressed by doping of the membrane with KOH. However, the use of KOH provides the opportunity for CO2 poisoning. For these reasons, conventional alkaline fuel cells typically operate on pure oxygen, or at least purified air.
The technology most widely used for CO2 removal is amine adsorption, but amine plants are complex and expensive. Membrane plants using CO2-selective cellulose acetate membranes have been in use since the 1980s and currently, the largest membrane facility for CO2 removal operates at 700 million scfd (standard cubic feet per day). Another known technology for CO2 removal is the morphysorb process which selectively removes acid gases, such as H2S, CO2, COS, and other components. Accordingly, there is a need for an anion exchange alkaline membrane fuel cell which is tolerant to CO2 so as to eliminate the need for CO2 removal.